Method and composition for improving corrosion resistance of zinc



United States Patent METHOD AND COMPOSIT ON FOR IMPROVING CORROSION RESISTANCE OF ZINC Allan E. Chester, Highland Park, Ill., assignor to Poor & Company, Chicago, Ill., a corporation of Delaware No Drawing. Application November 30, 1950, Serial No. 198,490

9 Claims. (Cl. 148-61) This invention relates to a method of improving the corrosion resistance of zinc surfaces and to a one-step process for effecting such improvement.

It is well known that zinc surfaces tend to turn white or grayish white with age, a property sometimes referred to as White corrosion, which is apparently due to the formation of oxides and carbonates. Several processes have been devised for increasing the resistance of zinc surfaces to white corrosion. One of these involves dipping the article made of zinc or containing a zinc coating in a chromic acid bath. The resultant product, however, discolors and therefore leaves much to be desired.

Another method of treating Zinc surfaces is the socalled bright dip wherein the zinc article or an article containing a zinc coating is dipped in a bath of chromic acid anhydride, sodium sulfate and nitric acid. This process has the disadvantage that the articles are hard to rinse and the bath has no substantial passivating effect on the zinc.

Improved brightness in articles containing an electroplated coating of zinc has been obtained by adding various types of addition agents to the electroplating bath. This process is also used in conjunction with a bright dip of the type previously described.

Several processes have been described in the literature which are said to produce a bright zinc plate. One of these involves the treatment of a Zinc plated article With a chromic acid bath containing a sulfate radical in a critical ratio followed by treatment in a second step with a solution of an acid having a pH less than 2.68. Another process involves a treatment in one step with a chromic acid bath and in a second step with a caustic alkali solution having a normality between 4.25 and saturation. Still a third process involves a treatment in which glacial acetic acid is employed with chromic acid in a first bath followed by a caustic dip in the second bath.

It will be observed that in many of the previously described processes the zinc surfaced article must be given a primary treatment and a secondary treatment.

One of the objects of the present invention is to provide a new and improved method of treating zinc articles or articles containing a surface coating of zinc to produce zinc surfaces which are resistant to white corrosion.

A further object is to provide a method of the character described which requires treatment of the zinc article or Zinc surfaced article in a single bath only, followed by rinsing.

An additional object is to provide new and useful compositions for protecting zinc.

Another object of the invention is to provide a chromic acid bath for treating zinc or Zinc surfaced articles which does not produce the brown discoloration normally characteristic of such baths. Other objects will appear hereinafter.

In accomplishing these objects in accordance with this step treatment and the products are satisfactory for many uses where greater protection is not required.

It is believed that the improved results obtained in the practice of the invention are due to the fact that the fluorine apparently whitens and brightens zinc and permits the chromic acid and nitric acid to attack the zinc surface uniformly. Also, it is believed that as zinc goes into solution the pH at the interface between the Zinc surface and the solution changes rapidly. In some unexplained manner fluorine compounds, as, for example, trifluoroacetic acid and fluoboric acid in the presence of nitric acid prevent the formation of brown chromium chromates and permit chromium oxides to form. Thus, the invention eliminates the necessity for a secondary operation to remove the brown chromium chromates which normally stain zinc surfaces that have been treated in chromic acid bath. The treatedzinc coatings produced in accordance with the invention are very receptive to paints and paint enamels whereas ordinary zinc surfaces cannot be painted satisfactorily.

The invention contemplates the preparation of the chromic acid and fluorine compounds, with or without auxiliary agents, as addition agents for nitric acid treating baths. The brightness of the resultant treated surface can be increased by increasing the concentration of the nitric acid. For example, the compositions of the invention may be added to a nitric acid bath containing 0.5 to 5% HNOs. Where brightness is not particularly desired, the concentration of HNOz may be relatively low, for example, 0.5 to 1.5% by Weight. 'On the other hand, if brightness is desired it can be obtained by employing 2% to 5% by weight of HNOa.

The invention will be illustrated but is not limited by the following example.

Example A composition was prepared by mixing together the following ingredients:

380 lbs. magnesium sulfate lbs. sodium dichromate 47.5 lbs. chromic acid 864 cc. trifluoroacetic acid This mixture was added to 200 gallons of water and agitated in a lead-lined tank. Additional water was added suflicient to make 396 gallons. The resultant composition was then added to carboys in the proportion of 99.5 lbs. for each carboy and to each carboy was also added 11.5 lbs. of fluoboric acid. The carboys were thencapped and stored.

If a stainless steel tank is used in mixing, the fluoboric acid can be added with the other ingredients, a total of 412.5 lbs. being required.

The chromatic acid content of the concentrate in this example is approximately 1.82 grams per gallon. The fluorine compounds content is approximately 1.59 grams per gallon, calculated as fluorine (F). F

In the previously described composition the magnesiu sulfate is employed to inhibit the attack of nitric acid on the zinc surface when the composition is employed Patented Dec. 20, 1955' as an addition agent to anitric acid bath as hereinafter described. V A v I The foregoing concentrate composition is employed as an addition agent to a nitric acid bath consisting of 0.5 to'5%' HNOQ. by weight dissolved, in water in proportions from 1:19 to' 1:99. by weight, preferably 1 part of concentrate to 49 parts of dilute HNOB.

The quantity of the addition agent employed may vary but is preferably such that the concentration of chromic acid- (.CrOa)? in the bath is within the range of about 1.82 grams per gallon to- 9.1 grams per gallon of bath. ltwillabe understood that the dichromate originally present in the addition agent is also'considered in calculating the chromic acid content of the bath.

Itwillbe understood that the fluoboricacid. may be replaced entirely by trifluoroacetic acid on an equivalent fluorine basis. It isalso possible to use all fluoboric acid. The fluorine content of the addition agent may range from about 1.59 to 7.95 grams per gallon of solution and it may be replaced wholly or in part by inorganic fluorine compounds, as, for example, sodium acid fluoride or sodium fluoride. Esters of fluoboric acidand trifluoroacetic acid can be used instead of the free acids, for example; the methyl, ethyl,-propyl and butyl esters as well as their higher homologues.

The relative proportions of thevarious. ingredients given inthe example are those which willproduce approximately optiffium' results. The chromicacid and fluorine contents ofthe treating bath are preferably approximately 3.64 and 3.18 grams per gallon, respectively.

' Because it contains a greater concentration of fluorine all'trifluoroacetic acid is preferably employed rather than a mixture of trifluoroacetic acid and fluoboric acid. However, in the commercial formula illustrated in the example fluoboric acid is used-becauseit providesa cheaper source of fluorine. It is generally less desirable to use fluorides because of their lower solubility in the bath and' the formation of hydrogen fluoride. Hydrogen fluoride can be used as such but sufiers from the disadvantage that it attacks glass and therefore compositions. containing it would require special types of containers. As'will be apparent, the fluorine containing acid formsonly a'fractionof a per cent by. weight of the treating bath.

In the'practice of. the invention thezinc surfaced article,

2 grams per liter of zinc dust are stirred into the electrolyte, and the entire mixture is filtered in order to remove traces of heavy metals.

The plated article is then rinsed and dried or without drying is immersed in a hath made by'addingthe previously described: addition'agent to a solution of 0.5 to 5% By weight HNOs. The bath is preferably at room temperature (75 degrees F.) and the article is immersed in the bath from 1 to seconds. Thereafter it is removed and thoroughly rinsed with water. It will be found that the corrosion resistance of the Zinc coating has been in creased and an article having an excellent appearance isobtained. As previously indicated; if it is desired to producea very bright zinc plate the quantity of nitric acidshould preferably be at least 2% by'weight of the bath.

The invention makes it possible te 'rev'ide a zinc surface whichis resis'tant to white corrosion and if desired" A typical con-' to produce a very bright zinc surface. This is accomplished by a simple one-step treatment which avoids the formation of brown, water soluble chromium chromate salts, eliminates the necessity for secondary operations to remove discoloring brown stains caused by such salts and at the same time provides a very uniform coating of enhanced brightness. The quantities of CrO3 required are relatively low. The coatings produced on zinc by the present invention not only are pleasing to the eye when allowed to remain as such, but also are receptive to paint and enamels and can be painted or enameled without the usual chipping or peeling that occurs when untreated zinc surfaces are painted or enameled.

It will be understood that the word enamel as used herein refers to paint enamels rather than vitreous enamels.

The expression zinc surfaced article is employed herein to cover any article having an exterior surface of zinc regardless of whether the article is made entirely of zinc or whether it has a Zinc coating. The expression zinc plated article is employed herein to describe a zinc coated article in which the zinc coating has been a plied by an electroplating process. I

The invention is hereby claimed as follows:

1. A process of protecting zinc against corrosion whichcomprises treating a zinc surfaced article with anaqueous solution consisting essentially of 0.5 to 5% by weight HNOs, 1 .82 to 9.1 grams per gallon of C10; and 1.59 to 7.95 grams per gallon of fluorine in the form of a water soluble fluorine-containing compound from the group con sisting of trifluoroacetic acid, fluoboric acid and thedovver.

alkyl esters of said acids.

2. A composition for treating zinc surfaces in order to protect the zinc against corrosion, said composition consisting essentially of an aqueous solution of chromic acid, nitric acid, fluoboric acid and trifluoroace'tic acid,-

the quantity of chromic acid being withinthe range of 1.82 to 9.1 grams per gallon, the quantity of nitric acid.

corresponding to 0.5 to 5% by weight,-and the quantity of said fluorine-containing acids calculated as fluorine being within the range from 1.59 to 7.95 grams per gallon.

3. An addition agent adaptedto be added to a nitric acid solution to provide a bathfor protecting zinc sin faces against corrosion consisting essentially of approximately 380 lbs. magnesium sulfate, approximately 1 lbs', sodium dichromate, approximately 47.5 lbs; chromic acid, and approximately 864' cc. trifluoroacetic acid dissolved in sulficient water to make approximately 396' gallons and approximately 412.5 lbs. fluoboric acidl 4. A process of protecting zinc against corrosion which comprises treating a zinc surfaced article with an aqueous solution consisting essentially of 0.5 to 5% by weight HNOz, 1.82- to 9.1 grams per gallon of CrOaarid 1.59 to 7.95 grams per gallon of fluorine in the form of'fluob'o'ric HNOs, 1.82 to 9.1 grams per gallon of CrOs and 1.59'

to 7.95 grams per gallon of fluorine in the form of trifiuor'oacetic acid.

6-. A process of protecting zinc against corrosion which comprises treating a'zinc surfaced article with: an aqueous solution consisting essentially of 0.5 to 5% by weight HNO3, 1.82 to 9.1 grams per gallon of CrOs and 1.59 to 7.95 grams per gallon of fluorine in the form of fluoboric acid and 'trifluoroacetic acid.

7. A composition for treating zinc surfaces consisting essentially of an aqueous solution of 0.5 to 5% by weight HNO 1.82 to 9.1 grams per gallono'f CrOs and 1.59 to 7.95 grams per'gallon of fluorine in the form of a water soluble fliioriiie containirig compoundfrom the group consisting of'trifluoro'ac'etic acid, fl'uoboric acid' aha the'Ioiver alky'l esters of said acids. 9

8. A composition for treating zinc surfaces in or'derto' protect the zinc against corrosion, said composition consisting essentially of an aqueous solution of 1.82 to 9.1 grams per gallon of CrOs, 0.5 to 5% by weight of HNO: and 1.59 to 7.95 grams per gallon of fluorine in the form of trifiuoroacetic acid, the relative proportions of said ingredients being sutficient to enhance the corrosion resistance of zinc while avoiding the formation of brown stains on the zinc surface when said composition is applied thereto.

9. An addition agent adapted to be added to a nitric acid solution to provide a bath for protecting zinc surfaces against corrosion consisting essentially of a chromium compound which forms chromic acid in the presence of an acid and a fluorine-containing compound from the group consisting of trifluoroacetic acid, fiuoboric acid and the lower alkyl esters of said acids, the proportions of said substances in said addition agent being such that said addition agent can be dissolved in water to produce a solution containing 1.82 to 9.1 grams per gallon of CrOs and 1.59 to 7.95 grams per gallon, calculated as fluorine, of said fluorine-containing compound.

References Cited in the file of this patent UNITED STATES PATENTS 2,288,007 Lum June 30, 1942 2,296,884 Thompson Sept. 29, 1942 2,376,158 McCarroll et al. May 15, 1945 2,428,749 De Long Oct. 7, 1947 2,471,909 Spruance, Jr May 31, 1949 2,483,510 Stareck Oct. 4, 1949 2,494,909 Spruance, Jr. et al I an. 17, 1950 2,524,577 Stareck Oct. 3, 1950 2,593,447 Hesch Apr. 22, 1952 2,593,448 Hesch Apr. 22, 1952 2,613,165 Fischer Oct. 7, 1952 FOREIGN PATENTS 229,616 Switzerland Feb. 1, 1944 

1. A PROCESS OF PROTECTING ZINC AGAINST CORROSION WHICH COMPRISES TREATIG A ZINC SURFACED ARTICLE WITH AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF 0.5 TO 5% BY WEIGHT HNO3, 1.82 TO 9.1 GRAMS PER GALLON OF CRO3 AND 1.59 TO 7.95 GRAMS PER GALLON OF FLUORINE IN THE FORM OF A WATER SOLUBLE FLUORINE-CONTAINING COMPOUND FROM THE GROUP CONSISTING OF TRIFLUOROATIC ACID, FLUOBORIC ACID AND THE LOWER ALKYL ESTER OF SAID ACIDS. 